1. Field of Invention
The present invention relates to a commercial device for removing NOx from a gas stream by employing microwave enhanced chemical reactions.
2. Background
Most gas streams, such as a diesel engine exhaust, contain many pollutants but always an important one is NOx. If combustion products are present then NOx and many other substances occur all of which require removal before release to the environment. Such pollutants include nitrogen oxides, carbon monoxide, particulate matter like soot, volatile organic compounds, and other hazardous air pollutants. In addition most combustion processes operate with considerable excess oxygen, usually from air--the likely source of the nitrogen which forms NOx--so that these combustion gases still contain residual oxygen, often in considerable amount.
In particular from a commercial viewpoint it is economically desirable to remove all combustion pollutants with one pass through an appropriate device and not have to employ successive apparatuses to fully accomplish the needed pollutant removal. The subject invention often accomplishes this with a selective bed employing appropriate catalysts along with astute microwave usage even though it is designed chiefly for NOx reduction.
A particularly difficult to remove combustion pollutant is nitrogen oxides present in various forms and usually identified as NOx to incorporate both NO and NO.sub.2. Microwave reduction of NOx proceeds well in the presence of pyrolytic carbon, such as char and soot, provided the oxygen content of the gas is small, generally less than 6 percent. As the oxygen content of the gas exceeds this 6 percent level, the removal of NOx becomes less and less efficient. Cha has shown this removal for the low oxygen situation in U.S. Pat. Nos. 5,246,554; 5,256,265; 5,269,892; and 5,362,451; and the specifications of these patents are hereby incorporated by reference. The high oxygen situation is covered by Cha's U.S. Pat. Nos. 5,767,470 and 6,027,698 whose specifications are hereby incorporated by reference. However the subject invention because of its unique design does not have any practical oxygen limitations as it works well under all oxygen conditions.
Quantum radiofrequency (RF) physics is based upon the phenomenon of resonant interaction with matter of electromagnetic radiation in the microwave and RF regions since every atom or molecule can absorb, and thus radiate, electromagnetic waves of various wavelengths. The rotational and vibrational frequencies of the electrons represent the most important frequency range. The electromagnetic frequency spectrum is conveniently divided into ultrasonic, microwave, and optical regions. The microwave region runs from 300 Mhz (megahertz) to 300 GHz (gigahertz) and encompasses frequencies used for much communication equipment. A treatise of such information is presented by Southworth, Principles and Applications of Waveguide Transmission, Nostrand, N.Y., 1950, which is herewith incorporated by reference.
Often the terms, microwave, microwaves and microwave energy are used interchangeably and are applied to a broad range of radiofrequency energies, such as 500 Mhz to 5000 MHz, particularly with respect to the common frequencies, 915 MHz and 2450 MHz. The former is often employed in industrial heating applications while the latter is the frequency of the common household microwave oven.
The absorption of microwaves by the energy bands, particularly the vibrational energy levels, of the atoms or molecules results in the thermal activation of the nonplasma material and the excitation of valence electrons. The nonplasma nature of these interactions is important for a separate and distinct form of heating employs plasma formed by arc conditions at a high temperature, often more than 3000.degree. F., and at much reduced pressures or vacuum conditions. For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Supplementary Volume, pages 599-608, Plasma Technology. In microwave technology, as applied in the subject invention, neither condition is present and therefore no plasmas are formed.
Microwaves lower the effective activation energy required for desirable chemical reactions since they can act locally on a microscopic scale by exciting electrons of a group of specific atoms in contrast to normal global heating which raises the bulk temperature. Further this microscopic interaction is favored by polar molecules whose electrons become easily locally excited leading to high chemical activity; however, nonpolar molecules adjacent to such polar molecules are also affected but at a much reduced extent. An example is the heating of polar water molecules in a common household microwave oven where the container is of nonpolar material, that is, microwave-passing, and stays relatively cool.
As used above microwaves are often referred to as a form of catalysis when applied to chemical reaction rates. For instance, see Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Volume 15, pages 494-517, Microwave Technology.
Related United States microwave patents include:
No. Inventor Year 4,935,114 Varma 1990 5,087,272 Nixdorf 1992 5,246,554 Cha 1993 5,256,265 Cha 1993 5,269,892 Cha 1993 5,362,451 Cha 1994 5,277,770 Murphy 1994 5,423,180 Nobue et al. 1995 5,536,477 Cha et al. 1996 5,767,470 Cha 1998 6,027,698 Cha 2000
Referring to the above list, Varma discloses a process for enhancing certain chemical reactions with microwaves; however, he only discloses laboratory equipment operating at above 400.degree. C. (752.degree. F.) and additionally has no variable flow control equipment. Conversely the subject invention requires careful control of the flowing gas as well as structural and temperature limits at about 500.degree. F. to produce an economically commercial apparatus.
Nixdorf discloses using a filter containing silicon carbide whiskers to remove particulate matter from a gas stream and then clean said filter with microwave heating. The subject invention is not just a filter.
Cha ('554) discloses removing gas oxides by adsorption on a char bed and then reduction by microwaves as two distinct steps. The subject invention does not employ radiofrequency catalysis using char.
Cha ('265) discloses removing gas oxides in a homogeneous mixture with soot carried out in a waveguide reactor. In contrast the subject invention does not actively employ homogeneous reduction involving soot and further has non-microwave conditions.
Cha ('892) discloses a pyrolytic carbon bed for removal of gas oxides using only microwave catalysis. The subject invention is not restricted to microwave catalysis.
Cha ('451) discloses a waveguide reactor to efficiently perform radiofrequency catalysis. The subject invention does not employ a waveguide reactor.
Murphy discloses reactivating plasma initiators using microwaves in the presence of oxygen, which is checked by a methane conversion reaction, where such plasma initiators are, or contain, metallic catalysts. The subject invention has no connection with the plasma regime of gases but does employ conventional metallic catalysts.
Nobue et al. disclose a filter regeneration system for an internal combustion engine using microwaves. The subject invention is not just a filter.
Cha et al. ('477) disclose a pollution arrestor using a soot filter followed by catalytic sections, using only reducing catalysts, to remove various gaseous pollutants with the total assembly within a microwave cavity. This pollution arrestor using only a reducing catalyst does not perform satisfactorily under high oxygen conditions. Conversely the subject invention performs well under all oxygen conditions, incorporates an oxidation/reduction catalyst rather than just a reducing catalyst, and employs a single flexible bed arrangement rather a series of multiple beds.
Cha ('470) discloses a process for pollution removal under high oxygen conditions using a series of beds containing various particle beds. Cha ('698) which is the device divisional of '470 discloses the apparatus for such a pollution removal, which has multiple beds with serial gas flow. Conversely the subject invention is based upon a single bed of varying particles that serves multiple functions when properly exposed to microwaves.